1. Field of the Invention
This invention relates to an ultrasonic Doppler blood flow velocity detection apparatus.
2. Description of the Prior Art
An ultrasonic Doppler blood flow velocity detection apparatus is used for detecting blood flow velocity which can be used in diagnoses. There are many types of Doppler blood flow detection apparatus utilizing Doppler effect through reflection of ultrasonic waves. The ultrasonic Doppler blood flow velocity detector can select a portion where blood flow is to be detected with respect to distance and direction.
The most popular ultrasonic Doppler blood flow velocity detection apparatus detects blood flow velocity as follows:
Such an ultrasonic Doppler blood flow velocity detection apparatus transmits an ultrasonic-wave pulse whose center frequency is "f", at a predetermined interval into the human body by a transducer; then it receives a reflected signal, i.e., an echo signal, from a moving reflective object, such as a blood corpuscle; and detects the amount of phase shift of the echo signal, i.e., Doppler shift. An output signal of the phase shift amount, i.e. Doppler signal, indicates blood flow velocity.
In this conventional Doppler blood flow velocity detection apparatus, the relation between a shift frequency fd of a Doppler signal and blood flow velocity V is given by: EQU fd=(2V/c).multidot.f cos .theta. (1)
where "c" is a sound velocity in the human body; .theta. is an angle made between the ultrasonic transmitting direction and the direction of blood flow, wherein the shift frequency fd is subjected to a limitation given by: EQU .vertline.fd.vertline..ltoreq.fp/2 (2)
where fp is a repetition frequency of ultrasonic-wave pulses (also referred to as a rate frequency).
The Doppler shift frequency fd should not exceed a half of the frequency fp because of the sampling theory. If blood velocity V exceeds a velocity corresponding to ultrasonic-wave pulse repetition frequency fp, the ultrasonic Doppler blood flow velocity detection apparatus outputs incorrect velocity and direction. Particularly, if a deep portion is measured, period of time from transmission of an ultrasonic-wave pulse to reception of the reflected ultrasonic waves by a sensor of the apparatus becomes long. Then, the frequency fp of the ultrasonic-wave pulse should be set at a low value. Therefore, it is difficult to detect a high velocity of blood flow.
An ultrasonic Doppler blood velocity detecting apparatus is disclosed in U.S. Pat. No. 4,534,357, which is provided to moderate the above-mentioned limitation of measurable blood flow velocity.
FIG. 7 is a block diagram of the above-mentioned disclosed ultrasonic Doppler blood velocity detecting apparatus. In FIG. 7, the device 210 of the apparatus includes standard Doppler system 212 (shown within dashed lines). The standard Doppler system 212 includes a master oscillator 214 and a transmitter 216 which transmits signals through a transducer 218. Signals received from the transducer 218 are passed through a receiver amplifier 220 and through a first multiplier 222 which multiplies the received signal by COS 2.pi.f1. The signal resulting from that multiplication goes through a low pass filter 224, a sample-and-hold circuit 226, and a high pass filter 228 into a mean frequency estimator 230. Similarly, the received signal from receiver 220 are passed through a multiplier 232 which multiplies them by SIN 2.pi.f1and the resulting signal is then sent through a low pass filter 234, a sample-and-hold circuit 236, and a high pass filter 238 into the mean frequency estimator 230.
The signals from the receiver 220 are also passed through a multiplier 242 where they are multiplied by COS 2.pi.f2. Those signals are then sent through a low pass filter 244, a sample-and-hold circuit 246, and a high pass filter 248 into a mean frequency estimator 250. Similarly, received signals from receiver 220 are multiplied in multiplier 252 by SIN 2.pi.f2 and sent into a low pass filter 254, a sample-and-hold circuit 256, and a high pass filter 258 into a mean frequency estimator 250. The outputs of the mean frequency estimators 230, 250 are sent into a subtractor 260 to obtain the unaliased mean frequency on output line 290.
Referring now to FIG. 8, the method of generating the sinusoidal waves corresponding to COS 2.pi.f1, SIN 2.pi.f1, SIN 2.pi.f2, and COS 2.pi.f2 on lines 223, 233, 253, and 243, respectively, are shown. In particular, outputs from the master oscillator 214, which is made to oscillate at a frequency of 4 nmp.times.PRF (pulse repetition frequency of ultrasonic signal), where n, m and p are integers, are sent into a divide-by-n circuit 262 to obtain an output frequency corresponding to 4f1 on a line 263. Similarly, outputs from the master oscillator 214 are sent through a divide-by-m circuit 274 to obtain a signal corresponding to 4f2 on line 275, and into a divide-by-4 nmp circuit 286 to obtain a signal on line 288 which corresponds to PRF.
As mentioned above, the Doppler blood velocity flow detection apparatus of U.S. Pat. No. 4,534,357 is provided for extending a measurable maximum blood flow velocity by compressing Doppler shift frequency by obtaining Doppler signal frequencies which are detected with different reference frequencies. However, there is a drawback that the frequency of the master oscillator becomes too high. For example, if Doppler signals of.+-.500 KHz is obtained from an echo signal having a center frequency of 5 MHz, f1=4.5 MHz and f2=5.5 MHz, the frequency of the master oscillator 214 should be 198 MHz because n=11 and m=9.
Ultrasonic Doppler blood flow velocity detection apparatus in currently used are so designed that their repetition frequency of ultrasonic pulses can be changed. For example, if three frequencies of ultrasonic pulses are provided for such an apparatus, three sets of reference signals are required in addition to repetition frequency for ultrasonic pulse generation. Therefore, six frequencies of reference signals are necessary. The frequency of the master oscillator would become further high because the frequency is determined by a factor four times a common denominator of these six frequencies.
In a diagnosis of blood flow velocity with the above-mentioned prior art ultrasonic Doppler blood flow velocity detection apparatus, the blood flow velocities which are different from each other in accordance with a portion of a human body to be measured. It is necessary to determine the frequencies f1 and f2 in accordance with a blood flow velocity. When a low velocity of blood flow is measured, a difference between f1 and f2 should be set relatively large; when a high velocity of a blood flow is measured, the difference between f1 and f2 to be small. Therefore, there is a drawback that the frequency of the master oscillator is required to have a very high frequency for providing flexibly setting of the frequencies f1 and f2.